1. Field of the Invention
The present invention relates to a wireless communication method and apparatus which perform media access control on the basis of carrier sense states.
2. Description of the Related Art
Media access control (MAC) is control for causing a plurality of communication apparatuses which perform communication while sharing the same medium to decide how to use the medium in transmitting communication data. Owing to media access control, even if two or more communication apparatuses transmit communication data by using the same medium at the same time, there is a lower possibility of a collision, in which a communication apparatus on the receiving side cannot separate communication data. Media access control also reduces the chance of a medium not being used by any communication apparatuses, despite the presence of communication apparatuses having transmission requests.
In wireless communication, since it is difficult for a communication apparatus to monitor transmission data while transmitting the data, media access control which is not premised on collision detection is required. IEEE 802.11, which is a typical technical standard for wireless local area networks (LANs), uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
According to CSMA/CA in IEEE 802.11, in the header of a MAC frame, a period (called a duration) until the end of a sequence comprising one or more frame exchanges following the MAC frame is set. In the duration, a communication apparatus which is irrelevant to the sequence and has no transmission right waits for transmission upon determining a virtual occupied state of the medium. This prevents collision. On the other hand, a communication apparatus which has a transmission right in this sequence recognizes that the medium is not used except for a period during which the medium is actually occupied.
IEEE 802.11 defines that the state of a medium is determined on the basis of a combination of virtual carrier sense on the MAC layer as in the former case and physical carrier sense on the physical layer as in the latter case, and media access control is performed on the basis of the determination.
Jpn. Pat. Appln. KOKAI Publication No. 2003-87856 discloses a method of realizing wireless base stations which can be commonly used in a plurality of wireless LAN schemes in a wireless communication system in which a plurality of wireless LAN schemes with different physical layers are present. More specifically, a wireless base station is made to alternately generate the first notification signal on the first physical layer and the second notification signal on the second physical layer and transmit them to wireless terminals, and the first and second physical layers are switched in synchronism with the first and second notification signals. The wireless terminal corresponding to the first physical layer is allowed access only for a predetermined period of time after the transmission time of the first notification signal, whereas the wireless terminal corresponding to the second physical layer is allowed access only for a predetermined period of time after the transmission time of the second notification signal.
IEEE 802.11 using CSMA/CA has made attempts to increase the communication speed mainly by changing the physical layer technology. With regard to the 2.4 GHz band, there have been changes from IEEE 802.11 (established in 1997, communication speed=2 Mbps) to IEEE 802.11b (established in 1999, communication speed=11 Mbps), and further to IEEE 802.11g (established in 2003, communication speed=54 Mbps). With regard to the 5 GHz band, only IEEE 802.11a (established in 1999, communication speed=54 Mbps) exists as a standard specifications.
In order to develop standard specifications directed to further increase communication speeds in both the 2.4- and the 5-GHz bands, IEEE 802.11 TGn (Task Group n) has already been established. In developing new specifications, when the same frequency band as that in the existing specifications (IEEE 802.11b/g/a) is to be used, it is important to assure coexistence with communication apparatuses conforming to the existing specifications. In addition, it is more preferable to have backward compatibility. For this reason, it is basically preferable that a protocol on the MAC layer conforms to CSMA/CA matching the existing specifications.
As an approach to an increase in communication speed, a method of increasing the frequency band of a channel is available. If a new specification uses a frequency band which has not been used, no problems arise in terms of coexistence and backward compatibility. According to Jpn. Pat. Appln. KOKAI Publication No. 2003-87856, although different frequency bands are set for the first and second physical layers, no frequencies overlap in the respective frequency bands, like for example, 4.9 to 5.0 GHz and 5.03 to 5.09 GHz. However, since frequencies are valuable resources, new channels having new frequency bands are preferably assigned to frequency bands which have already been used. For example, making one new channel include a plurality of existing channels is effective in increasing the frequency use efficiency.
Therefore, in order to allow communication apparatuses conforming to new specifications to maintain coexistence and backward compatibility with communication apparatuses conforming to existing specifications, it is necessary to perform not only media access control for new channels, at the time of communication using the new channels, but also media access control for existing channels by a method which can be understood by communication apparatuses conforming to the existing specifications. Conventional CSMA/CA is, however, designed to perform access control for one channel, and hence cannot perform media access control for a plurality of channels which have different frequency bandwidths and in which frequencies overlap.